A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. A speed reduction device such as an epicyclical gear assembly may be utilized to drive the fan section such that the fan section may rotate at a speed different than the turbine section so as to increase the overall propulsive efficiency of the engine. In such engine architectures, a shaft driven by one or more low pressure turbine sections provides an input to the epicyclical gear assembly that drives the fan section at a reduced speed such that both the turbine section and the fan section can rotate at closer to optimal speeds.
One type of gear assembly is referred to as a star gear assembly. In a star gear assembly a central or sun gear drives a plurality of intermediate or star gears driven that are supported by a fixed carrier. The star gears in turn drive a ring gear that drives the fan section. Because the carrier supporting the star gears remains fixed, a smaller load is encountered by bearings supporting rotation of the star gears due to lack of centrifugal forces. The reduced loads make roller elements practical instead of higher load capacity journal bearings. Although rolling element bearings are typically larger and heavier than journal bearings for the same load, they do not require an auxiliary lubrication system in case of oil interruption during flight. The weight savings by eliminating the auxiliary lubrication system offsets the weight increase of the larger rolling element bearings. Additionally, rolling element bearings can be used to offset the turbo-machinery thrust loads while journal bearings may not be capable of doing so. Lastly, the practical gear ratios available in a typical single stage star gear assembly are constrained by physical size limitations and these can be expanded to higher gear ratios by using a two stage compound star gear arrangement.
Although geared architectures have improved propulsive efficiency, turbine engine manufacturers continue to seek further improvements to engine performance including improvements to thermal, transfer and propulsive efficiencies.